1. Field of the Invention
This invention relates to digital electronic converters and particularly to methods and apparatus for converting digital signals into analog signals.
Converters between digital and analog domains are employed to interface between digital electronic circuitry and devices requiring analog signals. Accuracy of conversion, gain and repeatability in the process of conversion are matters of concern which are important to evaluate and for which compensation is frequently required.
Precision converters are needed for critical conversion applications. However, precision converters are generally expensive and difficult to implement. Reasonable cost, commercial quality converters frequently suffer from errors in conversion which render them unusable for critical applications. What is needed is a technique for increasing the accuracy of reasonably reliable commercially available converters so they can be used even in critical applications.
2. Description of the Prior Art
Trimming techniques for analog to digital and digital to analog converters have been suggested for limited purposes. For example, L. F. Pau, "Fast Testing and Trimming of A/D and D/A Converters in Automatic Test Systems", 1978, IEEE Auto Test Conference, has suggested a method for trimming converters based on the use of Walsh functions. A more generalized proposal for correction of converter errors based on Walsh functions is described in J. T. Millman, "Error Minimization of Data-Converting Systems via Generalized Spectral Analysis," Ph.D. Thesis, Massachusetts Institute of Technology, submitted Aug. 21, 1975 (available Oct. 27, 1975). Specifically, Millman suggests the use of on-the-converter error-correction circuitry employing Walsh functions which can be generated through the use of EXCLUSIVE OR functions under control of a programmable read only memory. Millman suggests incorporation of correction circuitry into the original design and layout of a digital to analog converter and specifically the provision of fine control of the current-setting resistor internal to the digital to analog converter. However, Millman recognized the infeasibility of implementing on-the-converter correction circuitry for any interactive Walsh function errors in existing circuitry. Millman's work has been limited to internal compensation and has made no suggestion for alternative solutions.
Intersil, Inc. of Cupertino, Calif. manufactures a correcting digital to analog converter device under the trade name ICL 7134. The ICL 7134 is a multiplying digital to analog converter which has a programmable read only memory array for controlling a 12 bit correcting digital to analog converter in the same device. The structure allows for modulo-32 cyclical corrections to be applied to the signal output. However, with the Intersil device, it is very difficult to correct for non-linearities in the analog output because all digital input functions are interrelated. Furthermore, only a group of the most significant bits are used for error correction so that significant errors involving the lower significant bits are potentially ignored.